The present invention relates to a booster shell reinforcing structure in a vacuum booster in which a booster shell provides: a front shell half having a circular flat portion, a tapered portion which is extended from the outer periphery of the circular flat plate portion and is made large in diameter backwardly, and a cylindrical portion extending backwardly from the rear edge of the tapered portion; and a rear shell half coupled to the rear end of the front shell half. In the vacuum booster, a reinforcement member has its outer peripheral portion corresponding to a coupling angular portion coupling between the flat plate portion and the tapered portion, the reinforcement member being laid on the inner surface of the flat plate portion. A flange of a master cylinder is joined to the outer surface of the flat plate portion with a plurality of bolts which are coupled to the reinforcement member and extend through the flat plate portion.
A conventional booster shell reinforcing structure of this type has been known, for instance, by Examined Japanese Utility Model Publication No. Sho. 60-21328.
In the front shell half, the flat plate portion and the tapered portion are coupled to each other with an angle formed between them; that is, there is provided a coupling angular portion coupling between the flat plate portion and the tapered portion. The coupling angular portion is liable to be bent back and forth by the load which is applied to the front wall of the front shell half during the operation of the vacuum booster and the master cylinder. In the conventional structure, the reinforcement member which is uniform in thickness and annular is laid on the inner surface of the flat plate portion in such a manner that its whole outer periphery corresponds in position to the coupling angular portion, whereby the whole peripheral portion of the coupling angular portion is increased in rigidity. The load applied to the front wall of the front shell half is greatest around the coupling bolts. Hence, the reinforcing of the front shell half can be sufficiently achieved by reinforcing only the portions of the front shell half with the reinforcement member which are around the coupling bolts.
However, in the conventional booster shell reinforcing structure, even the portions of the front shell half which are away from the coupling bolts are also reinforced with the reinforcement member large in thickness. This is not suitable for reduction of the weight of the vacuum booster.